Simplifications of cross-component linear model

ABSTRACT

A computing device performs a method of decoding video data by reconstructing a luma block corresponding to a chroma block; searching a sub-group of a plurality of reconstructed neighboring luma samples in a predefined order to identify a maximum luma sample and a minimum luma sample; computing a down-sampled maximum luma sample corresponding to the maximum luma sample; computing a down-sampled minimum luma sample corresponding to the minimum luma sample; generating a linear model using the down-sampled maximum luma sample, the down-sampled minimum luma sample, the first reconstructed chroma sample, and the second reconstructed chroma sample; computing down-sampled luma samples from luma samples of the reconstructed luma block, wherein each down-sampled luma sample corresponds to a chroma sample of the chroma block; and predicting chroma samples of the chroma block by applying the liner model to the corresponding down-sampled luma samples.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 17/700,238, filed on Mar. 21, 2022, which is acontinuation application of U.S. patent application Ser. No. 17/225,955,filed on Apr. 8, 2021, which is a continuation application of PCT PatentApplication No. PCT/US2019/055208, filed on Oct. 8, 2019, which claimsthe benefit of U.S. Provisional Patent Application No. 62/742,806, filedOct. 8, 2018, all of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present application generally relates to video data encoding anddecoding, and in particular, to method and system of reconstructing achroma block using a cross-component linear model during video dataencoding and decoding.

BACKGROUND

Digital video is supported by a variety of electronic devices, such asdigital televisions, laptop or desktop computers, tablet computers,digital cameras, digital recording devices, digital media players, videogaming consoles, smart phones, video teleconferencing devices, videostreaming devices, etc. The electronic devices transmit, receive,encode, decode, and/or store digital video data by implementing videocompression/decompression standards as defined by MPEG-4, ITU-T H.263,ITU-T H.264/MPEG-4, Part 10, Advanced Video Coding (AVC), HighEfficiency Video Coding (HEVC), and Versatile Video Coding (VVC)standard. Video compression typically includes performing spatial (intraframe) prediction and/or temporal (inter frame) prediction to reduce orremove redundancy inherent in the video data. For block-based videocoding, a video frame is partitioned into one or more slices, each slicehaving multiple video blocks, which may also be referred to as codingtree units (CTUs). Each CTU may contain one coding unit (CU) orrecursively split into smaller CUs until the predefined minimum CU sizeis reached. Each CU (also named leaf CU) contains one or multipletransform units (TUs) and each CU also contains one or multipleprediction units (PUs). Each CU can be coded in either intra, inter orIBC modes. Video blocks in an intra coded (I) slice of a video frame areencoded using spatial prediction with respect to reference samples inneighbor blocks within the same video frame. Video blocks in an intercoded (P or B) slice of a video frame may use spatial prediction withrespect to reference samples in neighbor blocks within the same videoframe or temporal prediction with respect to reference samples in otherprevious and/or future reference video frames.

Spatial or temporal prediction based on a reference block that has beenpreviously encoded, e.g., a neighbor block, results in a predictiveblock for a current video block to be coded. The process of finding thereference block may be accomplished by block matching algorithm.Residual data representing pixel differences between the current blockto be coded and the predictive block is referred to as a residual blockor prediction errors. An inter-coded block is encoded according to amotion vector that points to a reference block in a reference frameforming the predictive block, and the residual block. The process ofdetermining the motion vector is typically referred to as motionestimation. An intra coded block is encoded according to an intraprediction mode and the residual block. For further compression, theresidual block is transformed from the pixel domain to a transformdomain, e.g., frequency domain, resulting in residual transformcoefficients, which may then be quantized. The quantized transformcoefficients, initially arranged in a two-dimensional array, may bescanned to produce a one-dimensional vector of transform coefficients,and then entropy encoded into a video bitstream to achieve even morecompression.

The encoded video bitstream is then saved in a computer-readable storagemedium (e.g., flash memory) to be accessed by another electronic devicewith digital video capability or directly transmitted to the electronicdevice wired or wirelessly. The electronic device then performs videodecompression (which is an opposite process to the video compressiondescribed above) by, e.g., parsing the encoded video bitstream to obtainsyntax elements from the bitstream and reconstructing the digital videodata to its original format from the encoded video bitstream based atleast in part on the syntax elements obtained from the bitstream, andrenders the reconstructed digital video data on a display of theelectronic device.

With digital video quality going from high definition, to 4K×2K or even8K×4K, the amount of vide data to be encoded/decoded growsexponentially. It is a constant challenge in terms of how the video datacan be encoded/decoded more efficiently while maintaining the imagequality of the decoded video data.

SUMMARY

The present application describes implementations related to video dataencoding and decoding and, more particularly, to system and method ofreconstructing a chroma block using a cross-component linear modelduring video data encoding and decoding.

According to a first aspect of the present application, a method ofdecoding video data is performed at a computing device having one ormore processors and memory storing a plurality of programs to beexecuted by the one or more processors. A computing device performs amethod of reconstructing a luma block corresponding to a chroma block;searching a sub-group of a plurality of reconstructed neighboring lumasamples in a predefined order to identify a maximum luma sample and aminimum luma sample; computing a down-sampled maximum luma samplecorresponding to the maximum luma sample; computing a down-sampledminimum luma sample corresponding to the minimum luma sample; generatinga linear model using the down-sampled maximum luma sample, thedown-sampled minimum luma sample, the first reconstructed chroma sample,and the second reconstructed chroma sample; computing down-sampled lumasamples from luma samples of the reconstructed luma block, wherein eachdown-sampled luma sample corresponds to a chroma sample of the chromablock; and predicting chroma samples of the chroma block by applying theliner model to the corresponding down-sampled luma samples.

According to a second aspect of the present application, a computingdevice includes one or more processors, memory and a plurality ofprograms stored in the memory. The programs, when executed by the one ormore processors, cause the computing device to perform operations asdescribed above.

According to a third aspect of the present application, a non-transitorycomputer readable storage medium stores a plurality of programs forexecution by a computing device having one or more processors. Theprograms, when executed by the one or more processors, cause thecomputing device to perform operations as described above.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the implementations and are incorporated herein andconstitute a part of the specification, illustrate the describedimplementations and together with the description serve to explain theunderlying principles. Like reference numerals refer to correspondingparts.

FIG. 1 is a block diagram illustrating an exemplary video encoding anddecoding system in accordance with some implementations of the presentdisclosure.

FIG. 2 is a block diagram illustrating an exemplary video encoder inaccordance with some implementations of the present disclosure.

FIG. 3 is a block diagram illustrating an exemplary video decoder inaccordance with some implementations of the present disclosure.

FIGS. 4A-4D are block diagrams illustrating how a frame is recursivelyquad-tree partitioned into multiple video blocks of different sizes inaccordance with some implementations of the present disclosure.

FIG. 5A is a block diagram illustrating spatially neighboring andtemporally collocated block positions of a current CU to be encoded inaccordance with some implementations of the present disclosure.

FIG. 5B is a block diagram illustrating multi-threaded encoding ofmultiple rows of CTUs of a picture using wavefront parallel processingin accordance with some implementations of the present disclosure.

FIGS. 6A and 6B are block diagrams illustrating an exemplaryreconstructed luma block and an exemplary associated chroma block,respectively, in accordance with some implementations of the presentdisclosure.

FIGS. 7A-7E illustrate various ways of using the cross-component linearmodel to derive a linear model between luma values and chroma values inaccordance with some implementations of the present disclosure.

FIG. 8 is a flowchart illustrating an exemplary process by which a videocodec implements the techniques of using a cross-component linear modelto reconstruct chroma samples for a chroma block based on reconstructedluma samples from a luma block in accordance with some implementationsof the present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to specific implementations,examples of which are illustrated in the accompanying drawings. In thefollowing detailed description, numerous non-limiting specific detailsare set forth in order to assist in understanding the subject matterpresented herein. But it will be apparent to one of ordinary skill inthe art that various alternatives may be used without departing from thescope of claims and the subject matter may be practiced without thesespecific details. For example, it will be apparent to one of ordinaryskill in the art that the subject matter presented herein can beimplemented on many types of electronic devices with digital videocapabilities.

FIG. 1 is a block diagram illustrating an exemplary system 10 forencoding and decoding video blocks in parallel in accordance with someimplementations of the present disclosure. As shown in FIG. 1 , system10 includes a source device 12 that generates and encodes video data tobe decoded at a later time by a destination device 14. Source device 12and destination device 14 may comprise any of a wide variety ofelectronic devices, including desktop or laptop computers, tabletcomputers, smart phones, set-top boxes, digital televisions, cameras,display devices, digital media players, video gaming consoles, videostreaming device, or the like. In some implementations, source device 12and destination device 14 are equipped with wireless communicationcapabilities.

In some implementations, destination device 14 may receive the encodedvideo data to be decoded via a link 16. Link 16 may comprise any type ofcommunication medium or device capable of moving the encoded video datafrom source device 12 to destination device 14. In one example, link 16may comprise a communication medium to enable source device 12 totransmit the encoded video data directly to destination device 14 inreal-time. The encoded video data may be modulated according to acommunication standard, such as a wireless communication protocol, andtransmitted to destination device 14. The communication medium maycomprise any wireless or wired communication medium, such as a radiofrequency (RF) spectrum or one or more physical transmission lines. Thecommunication medium may form part of a packet-based network, such as alocal area network, a wide-area network, or a global network such as theInternet. The communication medium may include routers, switches, basestations, or any other equipment that may be useful to facilitatecommunication from source device 12 to destination device 14.

In some other implementations, the encoded video data may be transmittedfrom output interface 22 to a storage device 32. Subsequently, theencoded video data in storage device 32 may be accessed by destinationdevice 14 via input interface 28. Storage device 32 may include any of avariety of distributed or locally accessed data storage media such as ahard drive, Blu-ray discs, DVDs, CD-ROMs, flash memory, volatile ornon-volatile memory, or any other suitable digital storage media forstoring encoded video data. In a further example, storage device 32 maycorrespond to a file server or another intermediate storage device thatmay hold the encoded video data generated by source device 12.Destination device 14 may access the stored video data from storagedevice 32 via streaming or downloading. The file server may be any typeof computer capable of storing encoded video data and transmitting theencoded video data to destination device 14. Exemplary file serversinclude a web server (e.g., for a website), an FTP server, networkattached storage (NAS) devices, or a local disk drive. Destinationdevice 14 may access the encoded video data through any standard dataconnection, including a wireless channel (e.g., a Wi-Fi connection), awired connection (e.g., DSL, cable modem, etc.), or a combination ofboth that is suitable for accessing encoded video data stored on a fileserver. The transmission of encoded video data from storage device 32may be a streaming transmission, a download transmission, or acombination of both.

As shown in FIG. 1 , source device 12 includes a video source 18, avideo encoder 20 and an output interface 22. Video source 18 may includea source such as a video capture device, e.g., a video camera, a videoarchive containing previously captured video, a video feed interface toreceive video from a video content provider, and/or a computer graphicssystem for generating computer graphics data as the source video, or acombination of such sources. As one example, if video source 18 is avideo camera of a security surveillance system, source device 12 anddestination device 14 may form camera phones or video phones. However,the implementations described in the present application may beapplicable to video coding in general, and may be applied to wirelessand/or wired applications.

The captured, pre-captured, or computer-generated video may be encodedby video encoder 20. The encoded video data may be transmitted directlyto destination device 14 via output interface 22 of source device 12.The encoded video data may also (or alternatively) be stored ontostorage device 32 for later access by destination device 14 or otherdevices, for decoding and/or playback. Output interface 22 may furtherinclude a modem and/or a transmitter.

Destination device 14 includes an input interface 28, a video decoder30, and a display device 34. Input interface 28 may include a receiverand/or a modem and receive the encoded video data over link 16. Theencoded video data communicated over link 16, or provided on storagedevice 32, may include a variety of syntax elements generated by videoencoder 20 for use by video decoder 30 in decoding the video data. Suchsyntax elements may be included within the encoded video datatransmitted on a communication medium, stored on a storage medium, orstored a file server.

In some implementations, destination device 14 may include a displaydevice 34, which can be an integrated display device and an externaldisplay device that is configured to communicate with destination device14. Display device 34 displays the decoded video data to a user, and maycomprise any of a variety of display devices such as a liquid crystaldisplay (LCD), a plasma display, an organic light emitting diode (OLED)display, or another type of display device.

Video encoder 20 and video decoder 30 may operate according toproprietary or industry standards, such as VVC, HEVC, MPEG-4, Part 10,Advanced Video Coding (AVC), or extensions of such standards. It shouldbe understood that the present application is not limited to a specificvideo coding/decoding standard and may be applicable to other videocoding/decoding standards. It is generally contemplated that videoencoder 20 of source device 12 may be configured to encode video dataaccording to any of these current or future standards. Similarly, it isalso generally contemplated that video decoder 30 of destination device14 may be configured to decode video data according to any of thesecurrent or future standards.

Video encoder 20 and video decoder 30 each may be implemented as any ofa variety of suitable encoder circuitry, such as one or moremicroprocessors, digital signal processors (DSPs), application specificintegrated circuits (ASICs), field programmable gate arrays (FPGAs),discrete logic, software, hardware, firmware or any combinationsthereof. When implemented partially in software, an electronic devicemay store instructions for the software in a suitable, non-transitorycomputer-readable medium and execute the instructions in hardware usingone or more processors to perform the video coding/decoding operationsdisclosed in the present disclosure. Each of video encoder 20 and videodecoder 30 may be included in one or more encoders or decoders, eitherof which may be integrated as part of a combined encoder/decoder (CODEC)in a respective device.

FIG. 2 is a block diagram illustrating an exemplary video encoder 20 inaccordance with some implementations described in the presentapplication. Video encoder 20 may perform intra and inter predictivecoding of video blocks within video frames. Intra predictive codingrelies on spatial prediction to reduce or remove spatial redundancy invideo data within a given video frame or picture. Inter predictivecoding relies on temporal prediction to reduce or remove temporalredundancy in video data within adjacent video frames or pictures of avideo sequence.

As shown in FIG. 2 , video encoder 20 includes video data memory 40,prediction processing unit 41, decoded picture buffer (DPB) 64, summer50, transform processing unit 52, quantization unit 54, and entropyencoding unit 56. Prediction processing unit 41 further includes motionestimation unit 42, motion compensation unit 44, partition unit 45,intra prediction processing unit 46, and intra block copy (BC) unit 48.In some implementations, video encoder 20 also includes inversequantization unit 58, inverse transform processing unit 60, and summer62 for video block reconstruction. A deblocking filter (not shown) maybe positioned between summer 62 and DPB 64 to filter block boundaries toremove blockiness artifacts from reconstructed video. An in loop filter(not shown) may also be used in addition to the deblocking filter tofilter the output of summer 62. Video encoder 20 may take the form of afixed or programmable hardware unit or may be divided among one or moreof the illustrated fixed or programmable hardware units.

Video data memory 40 may store video data to be encoded by thecomponents of video encoder 20. The video data in video data memory 40may be obtained, for example, from video source 18. DPB 64 is a bufferthat stores reference video data for use in encoding video data by videoencoder 20 (e.g., in intra or inter predictive coding modes). Video datamemory 40 and DPB 64 may be formed by any of a variety of memorydevices. In various examples, video data memory 40 may be on-chip withother components of video encoder 20, or off-chip relative to thosecomponents.

As shown in FIG. 2 , after receiving video data, partition unit 45within prediction processing unit 41 partitions the video data intovideo blocks. This partitioning may also include partitioning a videoframe into slices, tiles, or other larger coding units (CUs) accordingto a predefined splitting structures such as quad-tree structureassociated with the video data. The video frame may be divided intomultiple video blocks (or sets of video blocks referred to as tiles).Prediction processing unit 41 may select one of a plurality of possiblepredictive coding modes, such as one of a plurality of intra predictivecoding modes or one of a plurality of inter predictive coding modes, forthe current video block based on error results (e.g., coding rate andthe level of distortion). Prediction processing unit 41 may provide theresulting intra or inter prediction coded block to summer 50 to generatea residual block and to summer 62 to reconstruct the encoded block foruse as part of a reference frame subsequently. Prediction processingunit 41 also provides syntax elements, such as motion vectors,intra-mode indicators, partition information, and other such syntaxinformation, to entropy encoding unit 56.

In order to select an appropriate intra predictive coding mode for thecurrent video block, intra prediction processing unit 46 withinprediction processing unit 41 may perform intra predictive coding of thecurrent video block relative to one or more neighbor blocks in the sameframe as the current block to be coded to provide spatial prediction.Motion estimation unit 42 and motion compensation unit 44 withinprediction processing unit 41 perform inter predictive coding of thecurrent video block relative to one or more predictive blocks in one ormore reference frames to provide temporal prediction. Video encoder 20may perform multiple coding passes, e.g., to select an appropriatecoding mode for each block of video data.

In some implementations, motion estimation unit 42 determines the interprediction mode for a current video frame by generating a motion vector,which indicates the displacement of a prediction unit (PU) of a videoblock within the current video frame relative to a predictive blockwithin a reference video frame, according to a predetermined patternwithin a sequence of video frames. Motion estimation, performed bymotion estimation unit 42, is the process of generating motion vectors,which estimate motion for video blocks. A motion vector, for example,may indicate the displacement of a PU of a video block within a currentvideo frame or picture relative to a predictive block within a referenceframe (or other coded unit) relative to the current block being codedwithin the current frame (or other coded unit). The predeterminedpattern may designate video frames in the sequence as P frames or Bframes. Intra BC unit 48 may determine vectors, e.g., block vectors, forintra BC coding in a manner similar to the determination of motionvectors by motion estimation unit 42 for inter prediction, or mayutilize motion estimation unit 42 to determine the block vector.

A predictive block is a block of a reference frame that is deemed asclosely matching the PU of the video block to be coded in terms of pixeldifference, which may be determined by sum of absolute difference (SAD),sum of square difference (SSD), or other difference metrics. In someimplementations, video encoder 20 may calculate values for sub-integerpixel positions of reference frames stored in DPB 64. For example, videoencoder 20 may interpolate values of one-quarter pixel positions,one-eighth pixel positions, or other fractional pixel positions of thereference frame. Therefore, motion estimation unit 42 may perform amotion search relative to the full pixel positions and fractional pixelpositions and output a motion vector with fractional pixel precision.

Motion estimation unit 42 calculates a motion vector for a PU of a videoblock in an inter prediction coded frame by comparing the position ofthe PU to the position of a predictive block of a reference frameselected from a first reference frame list (List 0) or a secondreference frame list (List 1), each of which identifies one or morereference frames stored in DPB 64. Motion estimation unit 42 sends thecalculated motion vector to motion compensation unit 44 and then toentropy encoding unit 56.

Motion compensation, performed by motion compensation unit 44, mayinvolve fetching or generating the predictive block based on the motionvector determined by motion estimation unit 42. Upon receiving themotion vector for the PU of the current video block, motion compensationunit 44 may locate a predictive block to which the motion vector pointsin one of the reference frame lists, retrieve the predictive block fromDPB 64, and forward the predictive block to summer 50. Summer 50 thenforms a residual video block of pixel difference values by subtractingpixel values of the predictive block provided by motion compensationunit 44 from the pixel values of the current video block being coded.The pixel difference values forming the residual vide block may includeluma or chroma difference components or both. Motion compensation unit44 may also generate syntax elements associated with the video blocks ofa video frame for use by video decoder 30 in decoding the video blocksof the video frame. The syntax elements may include, for example, syntaxelements defining the motion vector used to identify the predictiveblock, any flags indicating the prediction mode, or any other syntaxinformation described herein. Note that motion estimation unit 42 andmotion compensation unit 44 may be highly integrated, but areillustrated separately for conceptual purposes.

In some implementations, intra BC unit 48 may generate vectors and fetchpredictive blocks in a manner similar to that described above inconnection with motion estimation unit 42 and motion compensation unit44, but with the predictive blocks being in the same frame as thecurrent block being coded and with the vectors being referred to asblock vectors as opposed to motion vectors. In particular, intra BC unit48 may determine an intra-prediction mode to use to encode a currentblock. In some examples, intra BC unit 48 may encode a current blockusing various intra-prediction modes, e.g., during separate encodingpasses, and test their performance through rate-distortion analysis.Next, intra BC unit 48 may select, among the various testedintra-prediction modes, an appropriate intra-prediction mode to use andgenerate an intra-mode indicator accordingly. For example, intra BC unit48 may calculate rate-distortion values using a rate-distortion analysisfor the various tested intra-prediction modes, and select theintra-prediction mode having the best rate-distortion characteristicsamong the tested modes as the appropriate intra-prediction mode to use.Rate-distortion analysis generally determines an amount of distortion(or error) between an encoded block and an original, unencoded blockthat was encoded to produce the encoded block, as well as a bitrate(i.e., a number of bits) used to produce the encoded block. Intra BCunit 48 may calculate ratios from the distortions and rates for thevarious encoded blocks to determine which intra-prediction mode exhibitsthe best rate-distortion value for the block.

In other examples, intra BC unit 48 may use motion estimation unit 42and motion compensation unit 44, in whole or in part, to perform suchfunctions for Intra BC prediction according to the implementationsdescribed herein. In either case, for Intra block copy, a predictiveblock may be a block that is deemed as closely matching the block to becoded, in terms of pixel difference, which may be determined by sum ofabsolute difference (SAD), sum of squared difference (SSD), or otherdifference metrics, and identification of the predictive block mayinclude calculation of values for sub-integer pixel positions.

Whether the predictive block is from the same frame according to intraprediction, or a different frame according to inter prediction, videoencoder 20 may form a residual video block by subtracting pixel valuesof the predictive block from the pixel values of the current video blockbeing coded, forming pixel difference values. The pixel differencevalues forming the residual video block may include both luma and chromacomponent differences.

Intra prediction processing unit 46 may intra-predict a current videoblock, as an alternative to the inter-prediction performed by motionestimation unit 42 and motion compensation unit 44, or the intra blockcopy prediction performed by intra BC unit 48, as described above. Inparticular, intra prediction processing unit 46 may determine an intraprediction mode to use to encode a current block. To do so, intraprediction processing unit 46 may encode a current block using variousintra prediction modes, e.g., during separate encoding passes, and intraprediction processing unit 46 (or a mode select unit, in some examples)may select an appropriate intra prediction mode to use from the testedintra prediction modes. Intra prediction processing unit 46 may provideinformation indicative of the selected intra-prediction mode for theblock to entropy encoding unit 56. Entropy encoding unit 56 may encodethe information indicating the selected intra-prediction mode in thebitstream.

After prediction processing unit 41 determines the predictive block forthe current video block via either inter prediction or intra prediction,summer 50 forms a residual video block by subtracting the predictiveblock from the current video block. The residual video data in theresidual block may be included in one or more transform units (TUs) andis provided to transform processing unit 52. Transform processing unit52 transforms the residual video data into residual transformcoefficients using a transform, such as a discrete cosine transform(DCT) or a conceptually similar transform.

Transform processing unit 52 may send the resulting transformcoefficients to quantization unit 54. Quantization unit 54 quantizes thetransform coefficients to further reduce bit rate. The quantizationprocess may also reduce the bit depth associated with some or all of thecoefficients. The degree of quantization may be modified by adjusting aquantization parameter. In some examples, quantization unit 54 may thenperform a scan of a matrix including the quantized transformcoefficients. Alternatively, entropy encoding unit 56 may perform thescan.

Following quantization, entropy encoding unit 56 entropy encodes thequantized transform coefficients into a video bitstream using, e.g.,context adaptive variable length coding (CAVLC), context adaptive binaryarithmetic coding (CABAC), syntax-based context-adaptive binaryarithmetic coding (SBAC), probability interval partitioning entropy(PIPE) coding or another entropy encoding methodology or technique. Theencoded bitstream may then be transmitted to video decoder 30, orarchived in storage device 32 for later transmission to or retrieval byvideo decoder 30. Entropy encoding unit 56 may also entropy encode themotion vectors and the other syntax elements for the current video framebeing coded.

Inverse quantization unit 58 and inverse transform processing unit 60apply inverse quantization and inverse transformation, respectively, toreconstruct the residual video block in the pixel domain for generatinga reference block for prediction of other video blocks. As noted above,motion compensation unit 44 may generate a motion compensated predictiveblock from one or more reference blocks of the frames stored in DPB 64.Motion compensation unit 44 may also apply one or more interpolationfilters to the predictive block to calculate sub-integer pixel valuesfor use in motion estimation.

Summer 62 adds the reconstructed residual block to the motioncompensated predictive block produced by motion compensation unit 44 toproduce a reference block for storage in DPB 64. The reference block maythen be used by intra BC unit 48, motion estimation unit 42 and motioncompensation unit 44 as a predictive block to inter predict anothervideo block in a subsequent video frame.

FIG. 3 is a block diagram illustrating an exemplary video decoder 30 inaccordance with some implementations of the present application. Videodecoder 30 includes video data memory 79, entropy decoding unit 80,prediction processing unit 81, inverse quantization unit 86, inversetransform processing unit 88, summer 90, and DPB 92. Predictionprocessing unit 81 further includes motion compensation unit 82, intraprediction unit 84, and intra BC unit 85. Video decoder 30 may perform adecoding process generally reciprocal to the encoding process describedabove with respect to video encoder 20 in connection with FIG. 2 . Forexample, motion compensation unit 82 may generate prediction data basedon motion vectors received from entropy decoding unit 80, whileintra-prediction unit 84 may generate prediction data based onintra-prediction mode indicators received from entropy decoding unit 80.

In some examples, a unit of video decoder 30 may be tasked to performthe implementations of the present application. Also, in some examples,the implementations of the present disclosure may be divided among oneor more of the units of video decoder 30. For example, intra BC unit 85may perform the implementations of the present application, alone, or incombination with other units of video decoder 30, such as motioncompensation unit 82, intra prediction unit 84, and entropy decodingunit 80. In some examples, video decoder 30 may not include intra BCunit 85 and the functionality of intra BC unit 85 may be performed byother components of prediction processing unit 81, such as motioncompensation unit 82.

Video data memory 79 may store video data, such as an encoded videobitstream, to be decoded by the other components of video decoder 30.The video data stored in video data memory 79 may be obtained, forexample, from storage device 32, from a local video source, such as acamera, via wired or wireless network communication of video data, or byaccessing physical data storage media (e.g., a flash drive or harddisk). Video data memory 79 may include a coded picture buffer (CPB)that stores encoded video data from an encoded video bitstream. Decodedpicture buffer (DPB) 92 of video decoder 30 stores reference video datafor use in decoding video data by video decoder 30 (e.g., in intra orinter predictive coding modes). Video data memory 79 and DPB 92 may beformed by any of a variety of memory devices, such as dynamic randomaccess memory (DRAM), including synchronous DRAM (SDRAM),magneto-resistive RAM (MRAM), resistive RAM (RRAM), or other types ofmemory devices. For illustrative purpose, video data memory 79 and DPB92 are depicted as two distinct components of video decoder 30 in FIG. 3. But it will be apparent to one skilled in the art that video datamemory 79 and DPB 92 may be provided by the same memory device orseparate memory devices. In some examples, video data memory 79 may beon-chip with other components of video decoder 30, or off-chip relativeto those components.

During the decoding process, video decoder 30 receives an encoded videobitstream that represents video blocks of an encoded video frame andassociated syntax elements. Video decoder 30 may receive the syntaxelements at the video frame level and/or the video block level. Entropydecoding unit 80 of video decoder 30 entropy decodes the bitstream togenerate quantized coefficients, motion vectors or intra-prediction modeindicators, and other syntax elements. Entropy decoding unit 80 thenforwards the motion vectors and other syntax elements to predictionprocessing unit 81.

When the video frame is coded as an intra predictive coded (I) frame orfor intra coded predictive blocks in other types of frames, intraprediction unit 84 of prediction processing unit 81 may generateprediction data for a video block of the current video frame based on asignaled intra prediction mode and reference data from previouslydecoded blocks of the current frame.

When the video frame is coded as an inter-predictive coded (i.e., B orP) frame, motion compensation unit 82 of prediction processing unit 81produces one or more predictive blocks for a video block of the currentvideo frame based on the motion vectors and other syntax elementsreceived from entropy decoding unit 80. Each of the predictive blocksmay be produced from a reference frame within one of the reference framelists. Video decoder 30 may construct the reference frame lists, List 0and List 1, using default construction techniques based on referenceframes stored in DPB 92.

In some examples, when the video block is coded according to the intraBC mode described herein, intra BC unit 85 of prediction processing unit81 produces predictive blocks for the current video block based on blockvectors and other syntax elements received from entropy decoding unit80. The predictive blocks may be within a reconstructed region of thesame picture as the current video block defined by video encoder 20.

Motion compensation unit 82 and/or intra BC unit 85 determinesprediction information for a video block of the current video frame byparsing the motion vectors and other syntax elements, and then uses theprediction information to produce the predictive blocks for the currentvideo block being decoded. For example, motion compensation unit 82 usessome of the received syntax elements to determine a prediction mode(e.g., intra or inter prediction) used to code video blocks of the videoframe, an inter prediction frame type (e.g., B or P), constructioninformation for one or more of the reference frame lists for the frame,motion vectors for each inter predictive encoded video block of theframe, inter prediction status for each inter predictive coded videoblock of the frame, and other information to decode the video blocks inthe current video frame.

Similarly, intra BC unit 85 may use some of the received syntaxelements, e.g., a flag, to determine that the current video block waspredicted using the intra BC mode, construction information of whichvideo blocks of the frame are within the reconstructed region and shouldbe stored in DPB 92, block vectors for each intra BC predicted videoblock of the frame, intra BC prediction status for each intra BCpredicted video block of the frame, and other information to decode thevideo blocks in the current video frame.

Motion compensation unit 82 may also perform interpolation using theinterpolation filters as used by video encoder 20 during encoding of thevideo blocks to calculate interpolated values for sub-integer pixels ofreference blocks. In this case, motion compensation unit 82 maydetermine the interpolation filters used by video encoder 20 from thereceived syntax elements and use the interpolation filters to producepredictive blocks.

Inverse quantization unit 86 inverse quantizes the quantized transformcoefficients provided in the bitstream and entropy decoded by entropydecoding unit 80 using the same quantization parameter calculated byvideo encoder 20 for each video block in the video frame to determine adegree of quantization. Inverse transform processing unit 88 applies aninverse transform, e.g., an inverse DCT, an inverse integer transform,or a conceptually similar inverse transform process, to the transformcoefficients in order to reconstruct the residual blocks in the pixeldomain.

After motion compensation unit 82 or intra BC unit 85 generates thepredictive block for the current video block based on the vectors andother syntax elements, summer 90 reconstructs decoded video block forthe current video block by summing the residual block from inversetransform processing unit 88 and a corresponding predictive blockgenerated by motion compensation unit 82 and intra BC unit 85. Anin-loop filter (not pictured) may be positioned between summer 90 andDPB 92 to further process the decoded video block. The decoded videoblocks in a given frame are then stored in DPB 92, which storesreference frames used for subsequent motion compensation of next videoblocks. DPB 92, or a memory device separate from DPB 92, may also storedecoded video for later presentation on a display device, such asdisplay device 34 of FIG. 1 .

In a typical video coding process, a video sequence typically includesan ordered set of frames or pictures. Each frame may include threesample arrays, denoted SL, SCb, and SCr. SL is a two-dimensional arrayof luma samples. SCb is a two-dimensional array of Cb chroma samples.SCr is a two-dimensional array of Cr chroma samples. In other instances,a frame may be monochrome and therefore includes only onetwo-dimensional array of luma samples.

As shown in FIG. 4A, video encoder 20 (or more specifically partitionunit 45) generates an encoded representation of a frame by firstpartitioning the frame into a set of coding tree units (CTUs). A videoframe may include an integer number of CTUs ordered consecutively in araster scan order from left to right and from top to bottom. Each CTU isa largest logical coding unit and the width and height of the CTU aresignaled by the video encoder 20 in a sequence parameter set, such thatall the CTUs in a video sequence have the same size being one of128×128, 64×64, 32×32, and 16×16. But it should be noted that thepresent application is not necessarily limited to a particular size. Asshown in FIG. 4B, each CTU may comprise one coding tree block (CTB) ofluma samples, two corresponding coding tree blocks of chroma samples,and syntax elements used to code the samples of the coding tree blocks.The syntax elements describe properties of different types of units of acoded block of pixels and how the video sequence can be reconstructed atthe video decoder 30, including inter or intra prediction, intraprediction mode, motion vectors, and other parameters. In monochromepictures or pictures having three separate color planes, a CTU maycomprise a single coding tree block and syntax elements used to code thesamples of the coding tree block. A coding tree block may be an N×Nblock of samples.

To achieve a better performance, video encoder 20 may recursivelyperform tree partitioning such as binary-tree partitioning, quad-treepartitioning or a combination of both on the coding tree blocks of theCTU and divide the CTU into smaller coding units (CUs). As depicted inFIG. 4C, the 64×64 CTU 400 is first divided into four smaller CU, eachhaving a block size of 32×32. Among the four smaller CUs, CU 410 and CU420 are each divided into four CUs of 16×16 by block size. The two 16×16CUs 430 and 440 are each further divided into four CUs of 8×8 by blocksize. FIG. 4D depicts a quad-tree data structure illustrating the endresult of the partition process of the CTU 400 as depicted in FIG. 4C,each leaf node of the quad-tree corresponding to one CU of a respectivesize ranging from 32×32 to 8×8. Like the CTU depicted in FIG. 4B, eachCU may comprise a coding block (CB) of luma samples and twocorresponding coding blocks of chroma samples of a frame of the samesize, and syntax elements used to code the samples of the coding blocks.In monochrome pictures or pictures having three separate color planes, aCU may comprise a single coding block and syntax structures used to codethe samples of the coding block.

In some implementations, video encoder 20 may further partition a codingblock of a CU into one or more M×N prediction blocks (PB). A predictionblock is a rectangular (square or non-square) block of samples on whichthe same prediction, inter or intra, is applied. A prediction unit (PU)of a CU may comprise a prediction block of luma samples, twocorresponding prediction blocks of chroma samples, and syntax elementsused to predict the prediction blocks. In monochrome pictures orpictures having three separate color planes, a PU may comprise a singleprediction block and syntax structures used to predict the predictionblock. Video encoder 20 may generate predictive luma, Cb, and Cr blocksfor luma, Cb, and Cr prediction blocks of each PU of the CU.

Video encoder 20 may use intra prediction or inter prediction togenerate the predictive blocks for a PU. If video encoder 20 uses intraprediction to generate the predictive blocks of a PU, video encoder 20may generate the predictive blocks of the PU based on decoded samples ofthe frame associated with the PU. If video encoder 20 uses interprediction to generate the predictive blocks of a PU, video encoder 20may generate the predictive blocks of the PU based on decoded samples ofone or more frames other than the frame associated with the PU.

After video encoder 20 generates predictive luma, Cb, and Cr blocks forone or more PUs of a CU, video encoder 20 may generate a luma residualblock for the CU by subtracting the CU's predictive luma blocks from itsoriginal luma coding block such that each sample in the CU's lumaresidual block indicates a difference between a luma sample in one ofthe CU's predictive luma blocks and a corresponding sample in the CU'soriginal luma coding block. Similarly, video encoder 20 may generate aCb residual block and a Cr residual block for the CU, respectively, suchthat each sample in the CU's Cb residual block indicates a differencebetween a Cb sample in one of the CU's predictive Cb blocks and acorresponding sample in the CU's original Cb coding block and eachsample in the CU's Cr residual block may indicate a difference between aCr sample in one of the CU's predictive Cr blocks and a correspondingsample in the CU's original Cr coding block.

Furthermore, as illustrated in FIG. 4C, video encoder 20 may usequad-tree partitioning to decompose the luma, Cb, and Cr residual blocksof a CU into one or more luma, Cb, and Cr transform blocks. A transformblock is a rectangular (square or non-square) block of samples on whichthe same transform is applied. A transform unit (TU) of a CU maycomprise a transform block of luma samples, two corresponding transformblocks of chroma samples, and syntax elements used to transform thetransform block samples. Thus, each TU of a CU may be associated with aluma transform block, a Cb transform block, and a Cr transform block. Insome examples, the luma transform block associated with the TU may be asub-block of the CU's luma residual block. The Cb transform block may bea sub-block of the CU's Cb residual block. The Cr transform block may bea sub-block of the CU's Cr residual block. In monochrome pictures orpictures having three separate color planes, a TU may comprise a singletransform block and syntax structures used to transform the samples ofthe transform block.

Video encoder 20 may apply one or more transforms to a luma transformblock of a TU to generate a luma coefficient block for the TU. Acoefficient block may be a two-dimensional array of transformcoefficients. A transform coefficient may be a scalar quantity. Videoencoder 20 may apply one or more transforms to a Cb transform block of aTU to generate a Cb coefficient block for the TU. Video encoder 20 mayapply one or more transforms to a Cr transform block of a TU to generatea Cr coefficient block for the TU.

After generating a coefficient block (e.g., a luma coefficient block, aCb coefficient block or a Cr coefficient block), video encoder 20 mayquantize the coefficient block. Quantization generally refers to aprocess in which transform coefficients are quantized to possibly reducethe amount of data used to represent the transform coefficients,providing further compression. After video encoder 20 quantizes acoefficient block, video encoder 20 may entropy encode syntax elementsindicating the quantized transform coefficients. For example, videoencoder 20 may perform Context-Adaptive Binary Arithmetic Coding (CABAC)on the syntax elements indicating the quantized transform coefficients.Finally, video encoder 20 may output a bitstream that includes asequence of bits that forms a representation of coded frames andassociated data, which is either saved in storage device 32 ortransmitted to destination device 14.

After receiving a bitstream generated by video encoder 20, video decoder30 may parse the bitstream to obtain syntax elements from the bitstream.Video decoder 30 may reconstruct the frames of the video data based atleast in part on the syntax elements obtained from the bitstream. Theprocess of reconstructing the video data is generally reciprocal to theencoding process performed by video encoder 20. For example, videodecoder 30 may perform inverse transforms on the coefficient blocksassociated with TUs of a current CU to reconstruct residual blocksassociated with the TUs of the current CU. Video decoder 30 alsoreconstructs the coding blocks of the current CU by adding the samplesof the predictive blocks for PUs of the current CU to correspondingsamples of the transform blocks of the TUs of the current CU. Afterreconstructing the coding blocks for each CU of a frame, video decoder30 may reconstruct the frame.

As noted above, video coding achieves video compression using primarilytwo modes, i.e., intra-frame prediction (or intra-prediction) andinter-frame prediction (or inter-prediction). It is noted that IBC couldbe regarded as either intra-frame prediction or a third mode. Betweenthe two modes, inter-frame prediction contributes more to the codingefficiency than intra-frame prediction because of the use of motionvectors for predicting a current video block from a reference videoblock.

But with the ever improving video data capturing technology and morerefined video block size for preserving details in the video data, theamount of data required for representing motion vectors for a currentframe also increases substantially. One way of overcoming this challengeis to benefit from the fact that not only a group of neighboring CUs inboth the spatial and temporal domains have similar video data forpredicting purpose but the motion vectors between these neighboring CUsare also similar. Therefore, it is possible to use the motioninformation of spatially neighboring CUs and/or temporally collocatedCUs as an approximation of the motion information (e.g., motion vector)of a current CU by exploring their spatial and temporal correlation,which is also referred to as “motion vector predictor” (MVP) of thecurrent CU.

Instead of encoding, into the video bitstream, an actual motion vectorof the current CU determined by motion estimation unit 42 as describedabove in connection with FIG. 2 , the motion vector predictor of thecurrent CU is subtracted from the actual motion vector of the current CUto produce a motion vector difference (MVD) for the current CU. By doingso, there is no need to encode the motion vector determined by motionestimation unit 42 for each CU of a frame into the video bitstream andthe amount of data used for representing motion information in the videobitstream can be significantly decreased.

Like the process of choosing a predictive block in a reference frameduring inter-frame prediction of a code block, a set of rules need to beadopted by both video encoder 20 and video decoder 30 for constructing amotion vector candidate list (also known as a “merge list”) for acurrent CU using those potential candidate motion vectors associatedwith spatially neighboring CUs and/or temporally collocated CUs of thecurrent CU and then selecting one member from the motion vectorcandidate list as a motion vector predictor for the current CU. By doingso, there is no need to transmit the motion vector candidate list itselfbetween video encoder 20 and video decoder 30 and an index of theselected motion vector predictor within the motion vector candidate listis sufficient for video encoder 20 and video decoder 30 to use the samemotion vector predictor within the motion vector candidate list forencoding and decoding the current CU.

In some implementations, each inter-prediction CU has three motionvector prediction modes including inter (which is also referred to as“advanced motion vector prediction” (AMVP)), skip, and merge forconstructing the motion vector candidate list. Under each mode, one ormore motion vector candidates may be added to the motion vectorcandidate list according to the algorithms described below. Ultimatelyone of them in the candidate list is used as the best motion vectorpredictor of the inter-prediction CU to be encoded into the videobitstream by video encoder 20 or decoded from the video bitstream byvideo decoder 30. To find the best motion vector predictor from thecandidate list, a motion vector competition (MVC) scheme is introducedto select a motion vector from a given candidate set of motion vectors,i.e., the motion vector candidate list, that includes spatial andtemporal motion vector candidates.

In addition to deriving motion vector predictor candidates fromspatially neighboring or temporally collocated CUs, the motion vectorpredictor candidates can also be derived from the so-called“history-based motion vector prediction” (HMVP) table. The HMVP tablehouses a predefined number of motion vector predictors, each having beenused for encoding/decoding a particular CU of the same row of CTUs (orsometimes the same CTU). Because of the spatial/temporal proximity ofthese CUs, there is a high likelihood that one of the motion vectorpredictors in the HMVP table may be reused for encoding/decodingdifferent CUs within the same row of CTUs. Therefore, it is possible toachieve a higher code efficiency by including the HMVP table in theprocess of constructing the motion vector candidate list.

In some implementations, the HMVP table has a fixed length (e.g., 5) andis managed in a quasi-First-In-First-Out (FIFO) manner. For example, amotion vector is reconstructed for a CU when decoding one inter-codedblock of the CU. The HMVP table is updated on-the-fly with thereconstructed motion vector because such motion vector could be themotion vector predictor of a subsequent CU. When updating the HMVPtable, there are two scenarios: (i) the reconstructed motion vector isdifferent from other existing motion vectors in the HMVP table or (ii)the reconstructed motion vector is the same as one of the existingmotion vectors in the HMVP table. For the first scenario, thereconstructed motion vector is added to the HMVP table as the newest oneif the HMVP table is not full. If the HMVP table is already full, theoldest motion vector in the HMVP table needs to be removed from the HMVPtable first before the reconstructed motion vector is added as thenewest one. In other words, the HMVP table in this case is similar to aFIFO buffer such that the motion information located at the head of theFIFO buffer and associated with another previously inter-coded block isshifted out of the buffer so that the reconstructed motion vector isappended to the tail of the FIFO buffer as the newest member in the HMVPtable. For the second scenario, the existing motion vector in the HMVPtable that is substantially identical to the reconstructed motion vectoris removed from the HMVP table before the reconstructed motion vector isadded to the HMVP table as the newest one. If the HMVP table is alsomaintained in the form of a FIFO buffer, the motion vector predictorsafter the identical motion vector in the HMVP table are shifted forwardby one element to occupy the space left by the removed motion vector andthe reconstructed motion vector is then appended to the tail of the FIFObuffer as the newest member in the HMVP table.

The motion vectors in the HMVP table could be added to the motion vectorcandidate lists under different prediction modes such as AMVP, merge,skip, etc. It has been found that the motion information of previouslyinter-coded blocks stored in the HMVP table even not adjacent to thecurrent block can be utilized for more efficient motion vectorprediction.

After one MVP candidate is selected within the given candidate set ofmotion vectors for a current CU, video encoder 20 may generate one ormore syntax elements for the corresponding MVP candidate and encode theminto the video bitstream such that video decoder 30 can retrieve the MVPcandidate from the video bitstream using the syntax elements. Dependingon the specific mode used for constructing the motion vectors candidateset, different modes (e.g., AMVP, merge, skip, etc.) have different setsof syntax elements. For the AMVP mode, the syntax elements include interprediction indicators (List 0, List 1, or bi-directional prediction),reference indices, motion vector candidate indices, motion vectorprediction residual signal, etc. For the skip mode and the merge mode,only merge indices are encoded into the bitstream because the current CUinherits the other syntax elements including the inter predictionindicators, reference indices, and motion vectors from a neighboring CUreferred by the coded merge index. In the case of a skip coded CU, themotion vector prediction residual signal is also omitted.

FIG. 5A is a block diagram illustrating spatially neighboring andtemporally collocated block positions of a current CU to beencoded/decoded in accordance with some implementations of the presentdisclosure. For a given mode, a motion vector prediction (MVP) candidatelist is constructed by first checking the availability of motion vectorsassociated with the spatially left and above neighboring blockpositions, and the availability of motion vectors associated withtemporally collocated block positions and then the motion vectors in theHMVP table. During the process of constructing the MVP candidate list,some redundant MVP candidates are removed from the candidate list and,if necessary, zero-valued motion vector is added to make the candidatelist to have a fixed length (note that different modes may havedifferent fixed lengths). After the construction of the MVP candidatelist, video encoder 20 can select the best motion vector predictor fromthe candidate list and encode the corresponding index indicating thechosen candidate into the video bitstream.

Using FIG. 5A as an example and assuming that the candidate list has afixed length of two, the motion vector predictor (MVP) candidate listfor the current CU may be constructed by performing the following stepsin order under the AMVP mode:

-   -   1) Selection of MVP candidates from spatially neighboring CUs        -   a) Derive up to one non-scaled MVP candidate from one of the            two left spatial neighbour CUs starting with A0 and ending            with A1;        -   b) If no non-scaled MVP candidate from left is available in            the previous step, derive up to one scaled MVP candidate            from one of the two left spatial neighbour CUs starting with            A0 and ending with A1;        -   c) Derive up to one non-scaled MVP candidate from one of the            three above spatial neighbour CUs starting with B0, then B1,            and ending with B2;        -   d) If neither A0 nor A1 is available or if they are coded in            intra modes, derive up to one scaled MVP candidate from one            of the three above spatial neighbour CUs starting with B0,            then B1, and ending with B2;    -   2) If two MVP candidates are found in the previous steps and        they are identical, remove one of the two candidates from the        MVP candidate list;    -   3) Selection of MVP candidates from temporally collocated CUs        -   a) If the MVP candidate list after the previous step does            not include two MVP candidates, derive up to one MVP            candidate from the temporal collocated CUs (e.g., TO)    -   4) Selection of MVP candidates from the HMVP table        -   a) If the MVP candidate list after the previous step does            not include two MVP candidates, derive up to two            history-based MVP from the HMVP table; and    -   5) If the MVP candidate list after the previous step does not        include two MVP candidates, add up to two zero-valued MVPs to        the MVP candidate list.

Since there are only two candidates in the AMVP-mode MVP candidate listconstructed above, an associated syntax element like a binary flag isencoded into the bitstream to indicate that which of the two MVPcandidates within the candidate list is used for decoding the currentCU.

In some implementations, the MVP candidate list for the current CU underthe skip or merge mode may be constructed by performing a similar set ofsteps in order like the ones above. It is noted that one special kind ofmerge candidate called “pair-wise merge candidate” is also included intothe MVP candidate list for the skip or merge mode. The pair-wise mergecandidate is generated by averaging the MVs of the two previouslyderived merge-mode motion vector candidates. The size of the merge MVPcandidate list (e.g., from 1 to 6) is signaled in a slice header of thecurrent CU. For each CU in the merge mode, an index of the best mergecandidate is encoded using truncated unary binarization (TU). The firstbin of the merge index is coded with context and bypass coding is usedfor other bins.

As mentioned above, the history-based MVPs can be added to either theAMVP-mode MVP candidate list or the merge MVP candidate list after thespatial MVP and temporal MVP. The motion information of a previouslyinter-coded CU is stored in the HMVP table and used as a MVP candidatefor the current CU. The HMVP table is maintained during theencoding/decoding process. Whenever there is a non-sub-block inter-codedCU, the associated motion vector information is added to the last entryof the HMVP table as a new candidate while the motion vector informationstored in the first entry of the HMVP table is removed from therein (ifthe HMVP table is already full and there is no identical duplicate ofthe associated motion vector information in the table). Alternatively,the identical duplicate of the associated motion vector information isremoved from the table before the associated motion vector informationis added to the last entry of the HMVP table.

As noted above, intra block copy (IBC) can significantly improve thecoding efficiency of screen content materials. Since IBC mode isimplemented as a block-level coding mode, block matching (BM) isperformed at video encoder 20 to find an optimal block vector for eachCU. Here, a block vector is used to indicate the displacement from thecurrent block to a reference block, which has already been reconstructedwithin the current picture. An IBC mode is treated as the thirdprediction mode other than the intra or inter prediction modes.

At the CU level, the IBC mode can be signaled as IBC AMVP mode or IBCskip/merge mode as follows:

-   -   IBC AMVP mode: a block vector difference (BVD) between the        actual block vector of a CU and a block vector predictor of the        CU selected from block vector candidates of the CU is encoded in        the same way as a motion vector difference is encoded under the        AMVP mode described above. The block vector prediction method        uses two block vector candidates as predictors, one from left        neighbor and the other one from above neighbor (if IBC coded).        When either neighbor is not available, a default block vector        will be used as a block vector predictor. A binary flag is        signaled to indicate the block vector predictor index. The IBC        AMVP candidate list consists of spatial and HMVP candidates.    -   IBC skip/merge mode: a merge candidate index is used to indicate        which of the block vector candidates in the merge candidate list        (also known as a “merge list”) from neighboring IBC coded blocks        is used to predict the block vector for the current block. The        IBC merge candidate list consists of spatial, HMVP, and pairwise        candidates.

Another approach of improving the coding efficiency adopted by thestate-of-art coding standard is to introduce the parallel processing tothe video encoding/decoding process using, e.g., a multi-core processor.For example, wavefront parallel processing (WPP) has already beenintroduced into HEVC as a feature of encoding or decoding of multiplerows CTUs in parallel using multiple threads.

FIG. 5B is a block diagram illustrating multi-threaded encoding ofmultiple rows of CTUs of a picture using wavefront parallel processing(WPP) in accordance with some implementations of the present disclosure.When WPP is enabled, it is possible to process multiple rows of CTUs inparallel in a wavefront fashion, where there may be a delay of two CTUsbetween the start of two neighboring wavefronts. For example, to codethe picture 500 using WPP, a video coder, such as video encoder 20 andvideo decoder 30, may divide the coding tree units (CTUs) of the picture500 into a plurality of wavefronts, each wavefront corresponding to arespective row of CTUs in the picture. The video coder may start codinga top wavefront, e.g., using a first coder core or thread. After thevideo coder has coded two or more CTUs of the top wavefront, the videocoder may start coding a second-to-top wavefront in parallel with codingthe top wavefront, e.g., using a second, parallel coder core or thread.After the video coder has coded two or more CTUs of the second-to-topwavefront, the video coder may start coding a third-to-top wavefront inparallel with coding the higher wavefronts, e.g., using a third,parallel coder core or thread. This pattern may continue down thewavefronts in the picture 500. In the present disclosure, a set of CTUsthat a video coder is concurrently coding, using WPP, is referred to asa “CTU group.” Thus, when the video coder uses WPP to code a picture,each CTU of the CTU group may belong to a unique wavefront of thepicture and the CTU may be offset from a CTU in a respective, abovewavefront by at least two columns of CTUs of the picture.

The video coder may initialize a context for a current wavefront forperforming context adaptive binary arithmetic coding (CABAC) of thecurrent wavefront based on data of the first two blocks of the abovewavefront, as well as one or more elements of a slice header for a sliceincluding the first code block of the current wavefront. The video codermay perform CABAC initialization of a subsequent wavefront (or CTU row)using the context states after coding two CTUs of a CTU row above thesubsequent CTU row. In other words, before beginning coding of a currentwavefront, a video coder (or more specifically, a thread of the videocoder) may code at least two blocks of a wavefront above the currentwavefront, assuming the current wavefront is not the top row of CTUs ofa picture. The video coder may then initialize a CABAC context for thecurrent wavefront after coding at least two blocks of a wavefront abovethe current wavefront. In this example, each CTU row of the picture 500is a separated partition and has an associated thread (WPP Thread 1, WPPThread 2, . . . ) such that the number of CTU rows in the picture 500can be encoded in parallel.

Because the current implementation of the HMVP table uses a globalmotion vector (MV) buffer to store previously reconstructed motionvectors, this HMVP table cannot be implemented on the WPP-enabledparallel encoding scheme described above in connection with FIG. 5B. Inparticular, the fact that the global MV buffer is shared by all thethreads of the encoding/decoding process of a video coder prevents theWPP threads after the first WPP thread (i.e., WPP Thread 1) from beingstarted since these WPP threads have to wait for the HMVP table updatefrom the last CTU (i.e., rightmost CTU) of the first WPP thread (i.e.,the first CTU row) to be completed.

To overcome the problem, it is proposed that the global MV buffer sharedby the WPP threads be replaced with multiple CTU row-dedicated bufferssuch that each wavefront of CTU row has its own buffer for storing anHMVP table corresponding to the CTU row being processed by acorresponding WPP thread when WPP is enabled at the video coder. It isnoted that each CTU row having its own HMVP table is equivalent toresetting the HMVP table before coding a first CU of the CTU row. TheHMVP table reset is to flush out all the motion vectors in the HMVPtable resulting from coding of another CTU row. In one implementation,the reset operation is to set the size of the available motion vectorpredictors in the HMVP table to be zero. In yet another implementation,the reset operations could be to set the reference index of all theentries in the HMVP table to be an invalid value such as −1. By doingso, the construction of MVP candidate list for a current CTU within aparticular wavefront, regardless of which one of the three modes, AMVP,merge, and skip, is dependent upon an HMVP table associated with a WPPthread processing the particular wavefront. There is no inter-dependencybetween different wavefronts other than the two-CTU delay describedabove and the construction of motion vector candidate lists associatedwith different wavefronts can proceed in parallel like the WPP processdepicted in FIG. 5B. In other words, at the beginning of processing aparticular wavefront, the HMVP table is reset to be empty withoutaffecting the coding of another wavefront of CTUs by another WPP thread.In some cases, the HMVP table can be reset to be empty before the codingof each individual CTU. In this case, the motion vectors in the HMVPtable are limited to a particular CTU and there is probably a higherchance of a motion vector within the HMVP table being selected as amotion vector of a current CU within the particular CTU.

FIGS. 6A and 6B are block diagrams illustrating an exemplaryreconstructed luma block 602 and an exemplary associated chroma block620, respectively, in accordance with some implementations of thepresent disclosure. In this example, luma samples of reconstructed lumablock 602 (e.g., luma sample 604), top neighboring luma group 606 (e.g.,luma sample 608), and left neighboring luma group 610 (e.g., luma sample613) have been predicted during a video coding process. Chroma samplesof chroma block 620 are to be predicted, while chroma samples of topneighboring chroma group 624 (e.g., chroma sample 626) and leftneighboring chroma group 628 (e.g., chroma sample 630) have beenpredicted during the video coding process. In some embodiments, chromasamples of chroma block 620 can be predicted by applying a crosscomponent linear model (CCLM) to the corresponding down-sampled lumasamples of reconstructed luma block 602. Derivation and application ofthe CCLM is provided below in connection with FIGS. 7A-7E.

In some embodiments, reconstructed luma block 602 and chroma block 620each represent a different component of a portion of a reconstructedvideo frame. For example, in the YCbCr color space, an image isrepresented by a luma component (Y), a blue-difference chroma component(Cb), and a red-difference chroma component (Cr). Reconstructed lumablock 602 represents the luma component (i.e., brightness) of a portionof the video frame, and chroma block 620 represents a chroma component(i.e., color) of the same portion of the video frame. A luma sample(e.g., luma sample 604) of reconstructed luma block 602 has a luma valuerepresenting the brightness at a particular pixel of the video frame,and a chroma sample (e.g., chroma sample 622) has a chroma valuerepresenting the color at a particular pixel of the video frame.

In some embodiments, reconstructed luma block 602 is a 2M×2N block with2M luma samples across the block width and 2N luma samples across theblock height. M and N can be the same value (e.g., reconstructed lumablock 602 is a square block) or different values (e.g., reconstructedluma block 602 is a non-square block).

Chroma subsampling is a common compression technique as human visualsystem is less sensitive to the color difference than to brightnessdifference. As a result, reconstructed luma block 602 and chroma block620 may represent the same portion of a video frame but are encoded withdifferent resolutions. For example, the video frame may have beenencoded using a chroma subsampling scheme (e.g., 4:2:0 or 4:2:2) toencode for chroma information than for luma information with lessresolution. As illustrated in FIGS. 6A and 6B, reconstructed luma block602 is encoded with a resolution of 2M×2N, while chroma block 620 isencoded with a smaller resolution of M×N. In practice, chroma block 620can have other resolution such as 2M×2N (e.g., 4:4:4 full sampling),2M×N (e.g., 4:4:0 sub-sampling), M×2N (e.g., 4:2:2 sub-sampling), and{circumflex over ( )}1/2M×2N (e.g., 4:1:1 sub-sampling).

Reconstructed luma block 602 is next to top neighboring luma group 606and left neighboring luma group 610. The size of the top neighboringluma group and left neighboring luma group can be explicitly signaled ordependent upon the size of reconstructed luma block 602. For example,top neighboring luma group 606 can have a width of 2M samples (e.g.,same as the width of reconstructed luma block 602) or 4M samples (e.g.,double that of the width of reconstructed luma block 602), and a heightof 2 samples. Left neighboring luma group 610 can have a width of 2samples, with a height of 2N or 4N samples. In some embodiments, topneighboring luma group 606 and left neighboring luma group 610 are eacha portion of another luma block or blocks of the same video frame thathave been reconstructed.

Chroma block 620 is next to top neighboring chroma group 624 and leftneighboring group 628. The size of top neighboring chroma group 624 andleft neighboring group 628 can be explicitly signaled or dependent uponthe size of chroma block 620. For example, top neighboring chroma group624 can have a size of 1×M, and left neighboring chroma group 628 canhave a size of N× 1.

In some embodiments, chroma values (e.g., chroma values for chroma block620) can be predicted based on luma values of reconstructed luma samples(e.g., luma sample 604). For example, under the assumption that thereexists a linear or quasi-linear relation between luma values andcorresponding chroma values of a video frame, a video codec can predictchroma values based on corresponding reconstructed luma values using theCCLM. By doing so, the video codec can save a significant amount of timeand bandwidth for encoding the chroma values, transmitting the encodedchroma values, and decoding the encoded chroma values. To use the CCLMto predict chroma samples from luma samples, the video codec (1) derivesa linear model between the chroma samples and the luma samples, and (2)applies the linear model to reconstructed luma samples that correspondto the chroma samples to be predicted.

In some embodiments, since luma blocks and chroma blocks are ofdifferent resolutions (e.g., the chroma blocks having been sub-sampled),the video codec first performs down-sampling on luma samples to generatedown-sampled luma samples (e.g., down-sampled luma sample 605, 609, and612) that uniquely correspond to respective chroma samples. In someembodiments, six neighboring reconstructed luma samples in both heightand width direction of the video frame are used to generate adown-sampled luma sample (e.g., weighted-averaging schemes known in theart including six-tap down-sampling or like). For example, the sixreconstructed luma samples within region 611 (each represented by asmall box in the figure) inside top neighboring luma group are used togenerate down-sampled luma sample 609 through averaging of theircorresponding luma values, and the six reconstructed luma samples withinregion 607 (each represented by a small box in the figure) insidereconstructed luma block 602 are used to generate down-sampled lumasample 605. Alternatively, a down-sampled luma sample is generated byidentifying a reconstructed luma sample in a region of interest, orusing a different number of reconstructed luma samples in a region of adifferent shape.

In some embodiments, to drive the linear model, the video codec uses aMax-Min method by identifying maximum and minimum of down-sampled lumasamples (e.g., the down-sampled luma samples having the maximum and theminimum luma values, respectively) and the corresponding reconstructedchroma samples, and fitting a linear model (e.g., Y=αX+β) through themaximum and minimum data points (e.g., the maximum data point includesthe down-sampled luma sample having the maximum luma value and thecorresponding reconstructed chroma sample, and the minimum data pointincludes the down-sampled luma sample having the minimum luma value andthe corresponding reconstructed chroma sample). After the linear modelis derived, the video codec applies the linear model to down-sampledluma samples in reconstructed luma block 602 to generate correspondingchroma samples of chroma block 620. The video codec can obtain the maxand min data points in the following way:

1. In some embodiments, the video codec searches a group of down-sampledluma samples (e.g., a selected group of down-sampled luma samples in topneighboring luma group 606 and left neighboring luma group 610) toidentify the maximum down-sampled luma sample and the minimumdown-sampled luma sample. The video codec then identifiespreviously-reconstructed chroma samples (e.g., reconstructed chromasamples in top neighboring chroma group 624 and left neighboring chromagroup 628) corresponding to the maximum and minimum down-sampled lumasamples, as described below in connection with FIG. 7A and the relateddescription for detail on this implementation.

2. In some embodiments, the video codec searches a group ofreconstructed luma samples (e.g., a selected group of reconstructed lumasamples in top neighboring luma group 606 and left neighboring lumagroup 610) to identify (i) a reconstructed luma sample having themaximum luma value and (ii) a reconstructed luma sample having theminimum luma value in the selected group of reconstructed luma sampleswithout performing down-sampling on the selected group of reconstructedluma samples to identify the maximum and minimum reconstructed lumasamples. The video codec then performs down-sampling in a region (e.g.,a region with six samples using weighted-averaging schemes known in theart including six-tap down-sampling or like) associated with the maximumand the minimum reconstructed luma samples to generate a down-sampledluma sample as the maximum reconstructed luma sample (which may or maynot be exactly the maximum down-sampled luma sample) and a down-sampledluma sample as the min reconstructed luma sample (which may or may notbe exactly the minimum down-sampled luma sample). The video codec thenidentifies a reconstructed chroma sample (e.g., in top neighboringchroma group 624 and left neighboring chroma group 628) corresponding tothe down-sampled luma sample identified as the maximum reconstructedluma sample, and a reconstructed chroma sample corresponding to thedown-sampled luma sample identified as the minimum reconstructed lumasample, as described below in connection with FIG. 7B and the relateddescription for detail on this implementation.

3. In some embodiments, the video codec searches a group ofreconstructed chroma samples (e.g., selected chroma samples in topneighboring chroma group 624 and left neighboring chroma group 628) toidentify the maximum and the minimum reconstructed chroma samples (e.g.,the chroma samples having the maximum and minimum chroma values,respectively). The video codec then identifies the down-sampled lumasamples (e.g., down-sampled luma samples in top neighboring luma group606 and left neighboring luma group 610) corresponding to the maximumand min reconstructed chroma samples, as describe below in connectionwith FIG. 7C and the related description for detail on thisimplementation.

4. In some embodiments, the video codec searches a group of down-sampledluma samples (e.g., a selected group of down-sampled luma samples in topneighboring luma group 606 and left neighboring luma group 610) toidentify a predefined number (e.g., two) of down-sampled luma sampleshaving the largest luma values) and a predefined number (e.g., two) ofdown-sampled luma samples having the smallest luma values. The videocodec then identifies reconstructed chroma samples in top neighboringchroma group 624 and left neighboring chroma group 628, respectively,each corresponding to a respective one of the group of maximumdown-sampled luma samples and the group of minimum down-sampled lumasamples, respectively. The video codec then performs a weighted averageof values (e.g., chroma or luma values) within each of the identifiedgroups of reconstructed chroma and down-sampled luma samples to generatea maximum averaged chroma value, a minimum averaged chroma value, amaximum averaged down-sampled luma value (e.g., from the group ofmaximum down-sampled luma samples), and a minimum averaged down-sampledluma value (e.g., from the group of minimum down-sampled luma samples),as described below in connection with FIGS. 7D and 7E and the relateddescription for detail on this implementation.

FIGS. 7A-7E illustrate various approaches of using the CCLM to derive alinear model between luma values and chroma values in accordance withsome implementations of the present disclosure. In particular, eachcircle data point (e.g., point 702 a) on a plot represents a pair ofreconstructed chroma sample in the horizontal axis and a correspondingreconstructed luma sample in the vertical axis. For example, areconstructed chroma sample corresponds to a reconstructed luma sampleif a down-sampled luma sample generated in part using the reconstructedluma sample (e.g., using weighted-averaging schemes known in the artincluding six-tap down-sampling or like) corresponds to thereconstructed chroma sample. Each square data point (e.g., point 702 b)on the plot represents a pair of reconstructed chroma sample in thehorizontal axis and a corresponding down-sampled luma sample in thevertical axis. In some embodiments, since multiple reconstructed lumasamples are used to generate a down-sampled luma sample (e.g., using theweighted-averaging schemes known in the art including six-tapdown-sampling or like), a square data point is associated with multiplecircle data points. The dotted rectangle (e.g., the dotted rectangle703) indicates that the encompassed square data point and the circledata points are associated (e.g., the down-sampled luma samplecorresponding to the square data point is generated from thereconstructed luma samples corresponding to the circle data points). Forillustrative purpose, only one square data point and one circle datapoint are shown inside a dotted rectangle, although in actual practice,each dotted rectangle can include multiple circle data points and onesquare data point.

In some embodiments, the video codec searches the down-sampled lumasamples (e.g., down-sampled luma samples 609 and 612 of FIG. 6A) toidentify down-sampled luma samples having the maximum and minimum lumavalues and corresponding reconstructed chroma samples to derive thelinear model. For example, in FIG. 7A, square data point 702 brepresents a down-sampled luma sample having the minimum luma value ofthe selected group of down-sampled luma samples and a correspondingreconstructed chroma sample, and square data point 704 b represents adown-sampled luma sample having the maximum luma value and acorresponding reconstructed chroma sample. As a result, a line fittingthrough point 702 b and point 704 b represents the linear model topredict chroma samples (e.g., chroma sample 622 of chroma block 620 inFIG. 6B) from down-sampled luma samples (e.g., down-sampled luma sample605 of luma block 602 in FIG. 6A).

Generating the down-sampled luma samples is computationally intensive asmost of the down-sampled luma samples are used in a very little mannerin the Max-Min method. In some embodiments, instead of performingdown-sampling of all the luma samples, the video codec directly searchesreconstructed luma samples (e.g., reconstructed luma sample 608 and 613of FIG. 6A) to identify reconstructed luma samples having the maximumand minimum luma values. As illustrated in FIG. 7B, circle data point702 a represents a reconstructed luma sample having the minimum lumavalue and a corresponding reconstructed chroma sample, and circle datapoint 706 a represents a reconstructed luma sample having the maximumluma value and a corresponding reconstructed chroma sample. Afteridentifying the minimum and maximum reconstructed luma samples (circledata point 702 a and 706 a), the video codec then performs down-samplingin a region including the minimum and maximum reconstructed luma samples(e.g., using weighted-averaging schemes known in the art includingsix-tap down-sampling or like) to generate corresponding quasi minimumand maximum down-sampled luma samples (represented by square data points702 b and 706 b in FIG. 7B, which may or may not be the same as squaredata points 702 b and 704 b in FIG. 7A). A line fitting through squaredata points 702 b and 706 b in FIG. 7B represents the linear model topredict chroma samples from reconstructed luma samples. Compared to themethod used in FIG. 7A, only two down-sampling operations are performed.The identified min down-sampled luma sample is the same as the one usedin FIG. 7A, while the max down-sampled luma sample is different from theone used in FIG. 7A.

In some embodiments, the video codec generates the linear model usingthe reconstructed luma samples with the maximum and minimum luma values,and forgoes performing down-sampling. In FIG. 7C, no down-sampling isperformed on reconstructed luma samples, and a linear model is generatedby fitting a line through circle data point 702 a and 706 a directly.

In some embodiments, instead of searching for down-sampled luma samples(or reconstructed luma samples) with maximum and minimum luma values,the video codec first searches for reconstructed chroma samples havingthe maximum and minimum chroma values. Once the maximum and minimumreconstructed chroma samples are identified, the video codec thencomputes the corresponding down-sampled luma samples to generate thelinear model. In FIG. 7D, circle data point 708 a represents thereconstructed chroma sample having the minimum chroma value, and circledata point 704 a represents the reconstructed chroma sample having themaximum chroma value. The video codec then generates a linear modelfitting through square data point 708 b (which represents a down-sampledluma sample generated in part using the reconstructed luma sample fromcircle data point 708 a) and square data point 704 b (which represents adown-sampled luma sample generated in part using the reconstructed lumasample from circle data point 704 a).

In some embodiments, the video codec selects a number (e.g., two) ofdown-sampled luma samples having the largest luma values, and a number(e.g., two) of down-sampled luma samples having the smallest lumavalues. The video codec then and searches for the corresponding maximumand minimum groups of reconstructed chroma samples. The video codecperforms an averaging operation within each group, and uses the averagedluma and chroma values to generate a linear model. In FIG. 7E, thelargest two down-sampled luma samples (square data points 710 b and 704b) and the smallest two down-sampled luma samples (square data points708 b and square data points 702 b) are used to generate the linearmodel.

FIG. 8 is a flowchart illustrating an exemplary process 700 by which avideo codec implements the techniques of using a cross-component linearmodel to reconstruct chroma samples for a chroma block based onreconstructed luma samples from a luma block. Process 700 can beimplemented during either a decoding process or an encoding process.

As the first step, the video codec reconstructs a luma blockcorresponding to a chroma block (810). The chroma block (e.g., chromablock 620 of FIG. 6B) is to be subsequently reconstructed from thereconstructed luma block and may be of a different resolution differentfrom that of the reconstructed luma block (e.g., reconstructed lumablock 602 of FIG. 6A). The luma block corresponds to the chroma block asthey represent different components (e.g., brightness component andcolor component, respectively) of the same portion of a video frame. Insome embodiments, the luma block is adjacent to a plurality ofpreviously-reconstructed neighboring luma samples (e.g., reconstructedluma samples in top neighboring luma group 606 and left neighboring lumagroup 610 of FIG. 6A), and the chroma block is adjacent to a pluralityof previously-reconstructed neighboring chroma samples (e.g.,reconstructed chroma samples in top neighboring chroma group 624 andleft neighboring chroma group 628 of FIG. 6A). Note that the term“adjacent to” in this application is not limited to “immediately nextto” and also covers the situation in which the coding block is notimmediately next to the luma/chroma samples. In some embodiments, thevideo codec predicts chroma samples in the chroma block by deriving across-component linear model and applies the model to reconstructed lumasamples (or down-sampled reconstructed luma samples in the luma block).

Next, the video codec searches a sub-group of the plurality ofreconstructed neighboring luma samples in a predefined order to identifyat least one maximum or quasi-maximum luma sample and at least oneminimum or quasi-minimum luma sample (820). In some embodiments, thesub-group of the plurality of reconstructed neighboring luma samplescovers all the plurality of reconstructed neighboring luma samples. Forexample, the video codec can search all luma samples in the neighboringluma samples or up to a predefined number of neighboring luma samples.In some embodiments, the video codec searches the neighboring lumasamples according to the raster scan order, from left to right, from topto down, or in any combination of these orders. In some embodiments, theneighboring luma samples include those samples spatially on top of theluma block (e.g., top neighboring chroma group 624) and those samples tothe left of the luma block (e.g., left neighboring chroma group 610).The video codec may search only the top neighboring chroma samples orthe left neighboring chroma samples.

After identifying the maximum and minimum luma samples, the video codeccomputes at least one down-sampled maximum luma sample and at least onedown-sampled minimum luma sample corresponding to the identified maximumand minimum luma samples, respectively (830 and 840). For example, thevideo codec may use the six-tap down-sampling technique to generate adown-sampled luma sample (e.g., by weighted-averaging) from sixneighboring reconstructed luma samples (e.g., arranged in a 3×2 manneror a 2×3 manner). The down-sampled maximum luma sample and thedown-sampled minimum luma sample each correspond to a respectivereconstructed chroma sample (e.g., first reconstructed neighboringchroma sample and second reconstructed neighboring chroma sample,respectively). For example, the first reconstructed neighboring chromasample and the second reconstructed neighboring chroma sample can be ofthe plurality of reconstructed neighboring chroma samples.

In some implementations, the operations 820, 830, and 840 describedabove are re-arranged in a different order. For example, the video codecfirst computes one or more down-sampled luma samples from the pluralityof reconstructed neighboring luma samples and then searches, among asub-group of the one or more computed down-sampled luma samples, toidentify at least one down-sampled maximum luma sample and at least onedown-sampled minimum luma sample, respectively. The down-sampled maximumluma sample is chosen to correspond to the first reconstructed chromasample and the down-sampled minimum luma sample is chosen to correspondto the second reconstructed chroma sample.

Next, the video codec generates a linear model using a first pair of thedown-sampled maximum luma sample and the first reconstructed neighboringchroma sample and a second pair of the down-sampled minimum luma sampleand the second reconstructed neighboring chroma sample (850). In someembodiments, the video codec generates the linear model using themax-min method by identifying two data points (e.g., (firstreconstructed neighboring chroma sample, down-sampled maximum lumasample) and (second reconstructed neighboring chroma sample,down-sampled minimum luma sample)) and fits a linear equation throughthe two data points.

After obtaining the linear model, the video codec computes down-sampledluma samples from luma samples of the reconstructed luma block (860).Each down-sampled luma sample corresponds to a chroma sample of thechroma block. For example, the video codec can compute down-sampled lumasamples using the same down-sampling technique used to compute thedown-sampled max and min luma samples.

Finally, the video codec predicts chroma samples in the chroma block byapplying the linear model to the corresponding down-sampled luma samples(870).

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored on or transmitted over, as oneor more instructions or code, a computer-readable medium and executed bya hardware-based processing unit. Computer-readable media may includecomputer-readable storage media, which corresponds to a tangible mediumsuch as data storage media, or communication media including any mediumthat facilitates transfer of a computer program from one place toanother, e.g., according to a communication protocol. In this manner,computer-readable media generally may correspond to (1) tangiblecomputer-readable storage media which is non-transitory or (2) acommunication medium such as a signal or carrier wave. Data storagemedia may be any available media that can be accessed by one or morecomputers or one or more processors to retrieve instructions, codeand/or data structures for implementation of the implementationsdescribed in the present application. A computer program product mayinclude a computer-readable medium.

The terminology used in the description of the implementations herein isfor the purpose of describing particular implementations only and is notintended to limit the scope of claims. As used in the description of theimplementations and the appended claims, the singular forms “a,” “an,”and “the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise. It will also be understood that theterm “and/or” as used herein refers to and encompasses any and allpossible combinations of one or more of the associated listed items. Itwill be further understood that the terms “comprises” and/or“comprising,” when used in this specification, specify the presence ofstated features, elements, and/or components, but do not preclude thepresence or addition of one or more other features, elements,components, and/or groups thereof.

It will also be understood that, although the terms first, second, etc.may be used herein to describe various elements, these elements shouldnot be limited by these terms. These terms are only used to distinguishone element from another. For example, a first electrode could be termeda second electrode, and, similarly, a second electrode could be termed afirst electrode, without departing from the scope of theimplementations. The first electrode and the second electrode are bothelectrodes, but they are not the same electrode.

The description of the present application has been presented forpurposes of illustration and description, and is not intended to beexhaustive or limited to the invention in the form disclosed. Manymodifications, variations, and alternative implementations will beapparent to those of ordinary skill in the art having the benefit of theteachings presented in the foregoing descriptions and the associateddrawings. The embodiment was chosen and described in order to bestexplain the principles of the invention, the practical application, andto enable others skilled in the art to understand the invention forvarious implementations and to best utilize the underlying principlesand various implementations with various modifications as are suited tothe particular use contemplated. Therefore, it is to be understood thatthe scope of claims is not to be limited to the specific examples of theimplementations disclosed and that modifications and otherimplementations are intended to be included within the scope of theappended claims.

What is claimed is:
 1. A method for decoding a video signal, comprising:reconstructing a luma block corresponding to a chroma block, wherein theluma block is adjacent to a plurality of reconstructed neighboring lumasamples, and wherein the chroma block is adjacent to a plurality ofreconstructed neighboring chroma samples; identifying, from theplurality of reconstructed neighboring luma samples, two maximum lumasamples, wherein the two maximum luma samples correspond to two firstreconstructed chroma samples of the plurality of reconstructedneighboring chroma samples, respectively; identifying, from theplurality of reconstructed neighboring luma samples, two minimum lumasamples, wherein the two minimum luma samples correspond to two secondreconstructed chroma samples of the plurality of reconstructedneighboring chroma samples, respectively; averaging the two maximum lumasamples, the two minimum luma samples, the two first reconstructedchroma samples, and the two second reconstructed chroma samples,respectively, to obtain an averaged maximum luma sample, an averagedminimum luma sample, an averaged first reconstructed chroma sample andan averaged second reconstructed chroma sample; fitting a linear modelthrough the averaged maximum luma sample, the averaged minimum lumasample, the averaged first reconstructed chroma sample, and the averagedsecond reconstructed chroma sample; and predicting chroma samples of thechroma block by applying the linear model to the luma samples of theluma block.
 2. The method of claim 1, wherein the chroma block and theluma block are encoded using a 4:4:4 chroma full sampling scheme, andwherein the chroma block and the luma block have the same resolution. 3.The method of claim 1, wherein the plurality of reconstructedneighboring luma samples includes luma samples located above thereconstructed luma block and/or luma samples to left of thereconstructed luma block.
 4. The method of claim 1, wherein the fittingthe linear model comprises fitting a linear equation through one datapoint associated with the averaged maximum luma sample and the averagedfirst reconstructed chroma sample and one data point associated with theaveraged minimum luma sample and the averaged second reconstructedchroma sample.
 5. A coding device comprising: one or more processors;memory coupled to the one or more processors; and a plurality ofprograms stored in the memory that, when executed by the one or moreprocessors, cause the one or more processors to perform a method forcoding a video signal comprising: reconstructing a luma blockcorresponding to a chroma block, wherein the luma block is adjacent to aplurality of reconstructed neighboring luma samples, and wherein thechroma block is adjacent to a plurality of reconstructed neighboringchroma samples; identifying, from the plurality of reconstructedneighboring luma samples, two maximum luma samples, wherein the twomaximum luma samples correspond to two first reconstructed chromasamples of the plurality of reconstructed neighboring chroma samples,respectively; identifying, from the plurality of reconstructedneighboring luma samples, two minimum luma samples, wherein the twominimum luma samples correspond to two second reconstructed chromasamples of the plurality of reconstructed neighboring chroma samples,respectively; averaging the two maximum luma samples, the two minimumluma samples, the two first reconstructed chroma samples, and the twosecond reconstructed chroma samples, respectively, to obtain an averagedmaximum luma sample, an averaged minimum luma sample, an averaged firstreconstructed chroma sample and an averaged second reconstructed chromasample; fitting a linear model through the averaged maximum luma sample,the averaged minimum luma sample, the averaged first reconstructedchroma sample, and the averaged second reconstructed chroma sample; andpredicting chroma samples of the chroma block by applying the linearmodel to the luma samples of the luma block.
 6. The coding device ofclaim 5, wherein the chroma block and the luma block are encoded using a4:4:4 chroma full sampling scheme, and wherein the chroma block and theluma block have the same resolution.
 7. The coding device of claim 5,wherein the plurality of reconstructed neighboring luma samples includesluma samples located above the reconstructed luma block and/or lumasamples to left of the reconstructed luma block.
 8. The coding device ofclaim 5, wherein the fitting the linear model comprises fitting a linearequation through one data point associated with the averaged maximumluma sample and the averaged first reconstructed chroma sample and onedata point associated with the averaged minimum luma sample and theaveraged second reconstructed chroma sample.
 9. A non-transitorycomputer readable storage medium storing a plurality of programs forexecution by a computing device having one or more processors, whereinthe plurality of programs, when executed by the one or more processors,cause the computing device to receive bitstream and perform, based onthe bitstream, a method for decoding a video signal comprising:reconstructing a luma block corresponding to a chroma block, wherein theluma block is adjacent to a plurality of reconstructed neighboring lumasamples, and wherein the chroma block is adjacent to a plurality ofreconstructed neighboring chroma samples; identifying, from theplurality of reconstructed neighboring luma samples, two maximum lumasamples, wherein the two maximum luma samples correspond to two firstreconstructed chroma samples of the plurality of reconstructedneighboring chroma samples, respectively; identifying, from theplurality of reconstructed neighboring luma samples, two minimum lumasamples, wherein the two minimum luma samples correspond to two secondreconstructed chroma samples of the plurality of reconstructedneighboring chroma samples, respectively; averaging the two maximum lumasamples, the two minimum luma samples, the two first reconstructedchroma samples, and the two second reconstructed chroma samples,respectively, to obtain an averaged maximum luma sample, an averagedminimum luma sample, an averaged first reconstructed chroma sample andan averaged second reconstructed chroma sample; fitting a linear modelthrough the averaged maximum luma sample, the averaged minimum lumasample, the averaged first reconstructed chroma sample, and the averagedsecond reconstructed chroma sample; and predicting chroma samples of thechroma block by applying the linear model to the luma samples of theluma block.
 10. The non-transitory computer readable storage medium ofclaim 9, wherein the chroma block and the luma block are encoded using a4:4:4 chroma full sampling scheme, and wherein the chroma block and theluma block have the same resolution.
 11. The non-transitory computerreadable storage medium of claim 9, wherein the plurality ofreconstructed neighboring luma samples includes luma samples locatedabove the reconstructed luma block and/or luma samples to left of thereconstructed luma block.
 12. The non-transitory computer readablestorage medium of claim 9, wherein the fitting the linear modelcomprises fitting a linear equation through at least one data pointassociated with the at least one maximum luma sample and the firstreconstructed chroma sample and at least one data point associated withthe at least one minimum luma sample and the second reconstructed chromasample.